In this Letter, we demonstrate a high-temperature stable polymer planar waveguide Bragg grating based on cyclo-olefin copolymers. The high glass transition temperature of the polymer material amounting to 178°C, in conjunction with a high-temperature stable UV-curable adhesive used to connect the polymer sensor to a standard single-mode fiber, enables temperature readings of up to 160°C while exhibiting a temperature sensitivity of -7.3 pm/°C. The reflected power of the Bragg wavelength remains constant up to a temperature of 130°C before declining at higher temperatures with an overall reduction of 2.5 dB at 160°C. However, decreasing temperature results in a complete recovery of the peak power, facilitating steam pressure sterilization (129°C, 0.17 MPa) of the polymer planar waveguide Bragg grating.